This invention relates to hydraulic actuators for gate valves such as may be found in subsea Christmas trees used in petroleum production.
Typically, such actuators are hydraulically opened and include an integral, spring-operated fail-safe closure mechanism. In addition, the facility to manually operate the actuator in the event of a hydraulic failure is incorporated. This manual override is usually performed by an ROV that rotates an upper stem section of the actuator. The upper stem section is either threadingly mounted to the actuator body, or to a lower stem section coupled to the valve gate, so that rotation of the upper stem section is converted into linear motion of the lower stem section to open the valve.
At high production fluid pressures in the valve bore and cavity, the torque required to rotate the upper stem (and hence manually actuate the valve) is increased and can become excessive. There is also an inherent risk of galling and possible seizure of the mating threaded components.
The present invention provides a gate valve hydraulic actuator comprising:
a body;
a valve stem received in the body for linear sliding movement;
a piston connected for hydraulic actuation of the stem;
a stem extension accessible from outside the actuator body in use for imposition of linear movement thereto; and
locking dogs carried by or engageable with the stem or stem extension, operable selectively to lock the valve stem relative to the actuator body, to resist the linear sliding movement.
Preferably, the locking means are operable to lock the valve stem for such movement resistance in valve open and valve closed positions.
The locking dogs may be cam operated. Advantageously, the locking dogs are actuated by a cam spindle received in an axial bore formed in the stem extension.
The locking dogs may serve to lock the stem or stem extension to a collar surrounding the stem or stem extension and biassed towards the valve closed position by a spring positioned between the collar and the actuator body. This spring provides the valve fail-safe closure bias and, when the locking dogs are operative, also serves to resist the linear movement of the valve stem and/or stem extension, towards the valve open position.
Preferably the stem extension comprises an ROV linear tooling connector, such as a xe2x80x9cTorusxe2x80x9d type connector known in the industry.
Further preferred features and advantages of the invention are in the following description of an illustrative embodiment, made with reference to the drawings.